The present invention relates generally to a method of removing acidic components including, but not limited to, nitrogen oxide from waste gases and, more particularly, to such a method which may be used in conjunction with a power plant heated by coal wherein the waste gases, free of acidic components, are contacted in a gas phase with an oxidizing agent for the oxidation of oxides of nitrogen contained in the waste gases. The oxidizing agent subsequently absorbs the formed NO.sub.2 in an absorbing stage after addition of NH.sub.3. The thereby formed absorbing product is then oxidized with atmospheric oxygen to form ammonium nitrate in a subsequent oxidation stage and then, as a solution, is admitted to the evaporator.
The subject invention is an extension of current state of the art methods as described, for example, in West German Pat. No. DE-PS 25 18 386. In accordance with this prior art method, economical waste gas purification is conducted in a relatively simple manner wherein a dried salt is obtained while removing SO.sub.2 from the waste gases. This salt, which may be ammonium sulfate, may be used as fertilizer. Mud or other waste water problems which deleteriously affect the environment are therefore eliminated.
During combustion, waste gases from the furnaces are admitted in different concentrations in accordance with the types of furnaces, the types of boiler, sizes of boiler and the types of fuel used, etc. Generally speaking, the total NO.sub.x concentration may be between 200 and 2000 mg/Nm.sup.3. NO.sub.x is reduced to N.sub.2, which is removed with the waste gases free of SO.sub.x via a waste gas flue.
It is well known in the prior art as exemplified, for example, in West German patent application No. DE-PS 25 20 540, to remove NO.sub.x together with SO.sub.x from combustion waste gases. During desulfurization, the waste gases are washed with a solution containing ammonium sulfite which may afterwards be regenerated and reacted with coal gas containing NH.sub.3. In order to remove the NO.sub.x, the desulfurized waste gas is heated to a temperature between about 200.degree. C. and about 450.degree. C. and then passed over a catalytic agent bed which catalyzes the reaction of NO.sub.x with N.sub.3 to N.sub.2. During this procedure, additional energy is required for the reduction reaction. Furthermore, expensive catalytic agents must be used to eliminate potential environmental problems thus resulting in an expensive regeneration plant.
Another method of removing oxides of nitrates and sulfur oxides, from waste gases is described, for example, in West German patent application No. DE-OS 25 32 471 wherein the temperature of the NO.sub.x -containing waste gas is maintained above about 700.degree. C. The reaction occurs in the presence of ammonia and oxygen which thereby avoids the need for the use of a catalytic agent. This method is intended to avoid the formation of salts so that the walls of the air heater do not pollute the pipelines in the combustion furnace. In this procedure, no catalytic agent is used and no usable salt is obtained as an end product.
In accordance with the foregoing background, it is a primary object of the subject invention to provide a method of desulfurizing waste gases during combustion reactions.
It is a further object of the subject invention to provide a method wherein dried ammonium sulfate is obtained as a fertilizer during said desulfurization process.
It is still a further object of the subject invention to provide such a method wherein the NO.sub.2 contained in the waste gas is simultaneously removed to such an extent that a mixed commercial fertilizer consisting of ammonium sulfate and ammonium nitrate is obtained.
It is yet still a further object of the present invention to provide such a method wherein the mixing proportions of the two components may be adjusted to meet market demand.